Exploring quantum efficiency in cyanobacterial photosystem I: Influence of chlorophyll positioning and dissipation rates

Abstract

The efficiency of excitation energy transfer in photosystems can be quantified by the quantum efficiency q, defined as the fraction of incident excitation energy that initiates charge separation at the photosynthetic reaction-center. This efficiency can be calculated from a rate matrix that governs the stochastic migration of chlorophyll excitations within the photosystem. In this paper, we examined the resulting distributions of q in cyanobacterial photosystem I when chlorophylls are displaced randomly within the experimental spatial resolution of the photosystem structure and subjected to random rotational displacements. We also investigated how q varies with the internal dissipation rates of the chlorophylls. We found that q is largely unaffected by variations in the chlorophyll positions, which shows that photosynthesis in this photosystem is robust against structural perturbations. Meanwhile, the efficiency decreases sharply with increasing internal dissipation rate—photosynthesis is quickly suppressed even for small increases in dissipation rates, even if the dissipation rate is 10 times smaller than the rate of charge-separation at the P700.